In 1950 Cassen and his associates described an instrument that could measure the spatial distribution of radioactive iodine in the thyroid gland.1 Since that time, several different scanning devices have been developed and applied to problems in medical diagnosis.2-5

Advances have been chiefly along 3 lines: (1) the development of improved radiation detection equipment; (2) the production of radiopharmaceutical compounds that concentrate in organs, such as the liver and kidneys; and (3) a more complete understanding of the factors necessary to obtain good scanning images. It is now possible to construct scanning devices capable of detecting small differences in the spatial distribution of radioactivity in internal organs and presenting the data in a form that can be readily interpreted.

The photoscanner developed at the Johns Hopkins Hospital is used for numerous scanning procedures—for the detection of brain tumors,6 for thyroid scanning, for the differentiation of pericardial effusion